The present invention, in some embodiments thereof, relates to renewable energy, and, more particularly, but not exclusively, to a system and method for energy conversion based on phase-exchange processes.
According to thermodynamic principles, acoustic power in a gas is as valuable as other forms of work such as electrical power, rotating shaft power, and hydraulic power. For example, acoustic power can be used to produce refrigeration, such as in orifice pulse tube refrigerators; it can be used to produce electricity, via linear alternators; and it can be used to generate rotating shaft power, e.g., with a Wells turbine. Furthermore, acoustic power can be created from heat in a variety of heat engines such as Stirling engines and thermoacoustic engines.
Stirling's hot-air engine of the early 19th century was the first heat engine to use oscillating pressure and oscillating volume flow rate in a gas in a sealed system, although the time-averaged product thereof was not called acoustic power. It was subsequently recognized that the Stirling cycle could be reversed to produce useful cooling, if mechanical energy was provided to the system
Heretofore, a variety of engines and refrigerators related to the Stirling cycle have been developed. These include Stirling refrigerators, Ericsson engines, orifice pulse-tube refrigerators, standing-wave thermoacoustic engines and refrigerators, free-piston Stirling engines and refrigerators, and thermoacoustic-Stirling hybrid engines and refrigerators. Combinations thereof, such as the Vuilleumier refrigerator and the thermoacoustically driven orifice pulse tube refrigerator, have provided heat-driven refrigeration.
With the production ban of chlorofluorocarbons (CFC's), the interest in thermoacoustics has accelerated rapidly. For example, thermoacoustic refrigerators can be constructed such that they use only inert gases, which are non-toxic and do not contribute to ozone depletion, nor to global warming. Exemplary thermoacoustic engines and refrigerators are disclosed in U.S. Pat. Nos. 4,398,398; 4,489,553, 4,722,201, 5,303,555, 5,647,216, 5,953,921, 6,032,464, and 6,314,740, the contents of which are hereby incorporated by reference.
In a traditional Stirling engine, for example, high-temperature heat is added to the engine at a hot heat exchanger, and ambient-temperature waste heat is removed from the engine at an ambient heat exchanger. A solid matrix (also known as a regenerator) containing small pores having gas therein smoothly spans the temperature difference between the hot heat exchanger and the ambient heat exchanger. The temperature gradient across the regenerator provides the conditions for generation of gas oscillations within the pores, thus converting the heat entered at the hot heat exchanger into acoustic power. The gas in the pores of the regenerator moves toward the hot heat exchanger while the pressure is high and toward the ambient heat exchanger while the pressure is low. The oscillating thermal expansion and contraction of the gas, attending its oscillating motion along the temperature gradient in the pores, is therefore temporally phased with respect to the oscillating pressure so that the thermal expansion occurs while the pressure is high and the thermal contraction occurs while the pressure is low.